/Engine/Source/Editor/UnrealEd/Public/EditorViewportClient.h
Using commits a single usage record for whichever category is sent to it only if it's different from the last category that was sent to Using. This should be used to capture usage data for modes where it's difficult to tell when they ended.
static void Using ( [Category](API\Editor\UnrealEd\FEditorViewportStats\Category) InCategory )